A semiconductor laser for a light source to be used for optical communication is demanded to oscillate stably with a single wavelength. Further, from a point of view of low power consumption, also it is demanded for the semiconductor laser to have low oscillation threshold current.
Conventionally, as a semiconductor laser which oscillates stably with a single wavelength, a distributed feedback (DFB) laser having a phase shift is used.
In a phase shift DFB laser, in order to implement low threshold value operation, the coupling coefficient (diffraction strength) of a diffraction grating is set high so that great feedback is obtained.
However, where, as shown in FIG. 1(A), a phase shift (here, a λ/4 phase shift) 11 is provided at the center of a diffraction grating 10 of a DFB laser, if the coupling coefficient is set to a great value, then concentration of light intensity (photoelectric field intensity) occurs in the proximity of the phase shift 11 as seen in FIG. 1(B) and the stimulated emission rate in the proximity of the phase shift 11 becomes high as a result of high light intensity and electron-hole pairs (carriers) decrease.
As a result, the carrier density becomes comparatively low in the proximity of the phase shift 11 while the carrier density increases at end portions of the diffraction grating 10 as seen in FIG. 1(C), resulting in non-uniformity of the carrier density.
Here, the carrier density has, by a plasma effect thereof, an influence on the refractive index (waveguide refractive index) of a semiconductor material of which an optical waveguide of a laser is made.
Therefore, if non-uniformity of the carrier density appears, then non-uniformity of the waveguide refractive index appears. In particular, since the carrier density is low in the proximity of the phase shift 11, the waveguide refractive index is high, but, since the carrier density is high at the end portions of the diffraction grating 10, the waveguide refractive index is low.
Such a difference of the waveguide refractive index as just described is equal to that of the optical length of the optical waveguide and has an influence on the Bragg wavelength. In particular, since the refractive index is high in the proximity of the phase shift 11 as seen in FIG. 1(D), the optical length becomes long and the wavelength of the Bragg wavelength becomes long. However, at the end portions, since the refractive index is low, the optical length becomes short and the Bragg wavelength becomes short.
As a result, the Bragg wavelengths do not coincide in the resonator. Therefore, if the injection current value is increased so that a desired optical output is obtained, then the oscillation spectrum of the laser degrades and stable single-mode operation (single-wavelength operation) cannot be implemented any more. Such a phenomenon as just described is called spatial hole burning (for example, refer to Soda et al., “Stability in Single Longitudinal Mode Operation in GaInAsP/InP Phase-Adjusted DFB Lasers”, IEEE Journal of Quantum Electronics, vol. QE-23, No. 6, June 1987, pp. 804-814).
Here, FIG. 2 illustrates oscillation spectrums where the injection current value is successively increased like 7 mA, 10 mA, 20 mA, 40 mA, 60 mA, 80 mA, and 100 mA (the injection current value is greater on the upper side in FIG. 2).
As seen in FIG. 2, it is recognized that, if the injection current value is increased so that a desired optical output is obtained, then light intensity increases and multi-mode oscillation occurs, and as a result, single-mode operation cannot be implemented anymore.